The present invention relates to plasma processing technology. In particular the invention pertains to treatment of surfaces by a flow of plasma produced atomic or molecular medium excited to metastable level to clean the surfaces from undesirable contaminating residues, to improve surface adhesive bonding, to provide surface sterilization, or to modify selectively the chemical and crystalline structure of the surface treated for semiconductor, display, micromachining, and medical related technologies.
There exists a plurality of methods of plasma treatment of solid surfaces for the purpose of cleaning, etching, and enhancing these surfaces. Within these methods, the plasma cleaning technologies form a separate group of treatments. Within this separate group there is a distinctive difference between passive plasma cleaning processes, when the surfaces treated are exposed passively to plasma or its products, and active ones, when the surfaces treated interact as electrically biased electrodes with charged components of plasma (electrons and ions). A description and a classification of plasma cleaning processes is presented in xe2x80x9cIndustrial Plasma Engineeringxe2x80x9d, Vol. 2, Applications to Nonthermal Plasma Processing, by J. Reece Roth, Institute of Physics Publishing, 2001, p.p. 341-359. The present invention relates closely to the passive plasma cleaning processes. More specifically, it relates to surface modification processes provided by a neutral long living metastable component excited in plasmas of a working gaseous medium
As it is well known from gas discharge laser technology, a significant concentration of excited atoms or molecules of the working gas can be achieved in gas discharge plasmas and used as an active medium for laser generation, see for example: xe2x80x9cKinetic Processes in Gases and Molecular Lasersxe2x80x9d by B. F. F. Gordiets, A. I. Osipov, L. A. Shelepin, Gordon and Breach Publishing Group, 1988. The long living metastable states of the electrons in the atoms (or molecules) can exist due to high quantum symmetry of these metastable states provided by atomic (or molecular) electron configuration. Since interaction of the excited atoms (or molecules) with a surface violates this symmetry, the energy of excitation is liberated (with a final electron transition to ground level) and is absorbed in the surface of the solid with significant probability. This effect of originating an anomalous thermo-conductivity in gases of gas discharge lasers is described in a paper by E. V. Shun""ko: xe2x80x9cSome features of power balance in gas discharge in CO2, N2, and their mixtures with Hexe2x80x9d, J. Appl. Phys., 70(12), 1991, p.p. 7273-7281.
1. Thus the atoms (or molecules) excited to metastable level can be utilized for modification of the surfaces of the solids.
2. The complete electron transition energy to ground level for certain metastable excited atoms can achieve the value xcx9c15 eV which is enough to destruct structural bonds in the surface of the work piece and increase in this manner the surface energy above 70 erg/cm2 (surface tension of water at the room temperature) improving surface adhesive bonding significantly.
3. The relatively high energy xcx9c15 eV, which can be liberated from each electron transition in the process of metastable atom relaxation on the surface, enables one to form the free radicals in a biomaterial contaminating the surface that provides in this manner a surface sterilization.
4. Choosing necessary operating gas or gas composition within available gases and vapors, and choosing in this manner a necessary energy of the operating metastable level, one can selectively induce or disintegrate chemical bonds of the material coating the surface.
5. The possible long life time of xcfx84m greater than 0.001 sec (for example) realized for the metastable state of some kinds of atoms or molecules enables one to separate excited atoms from plasma by blowing operating gas through a discharge gap with the average flow velocity of vg greater than 10 m/s, providing a physical disconnection of the operating gas saturated with the excited atoms from a plasma boundary at the distance d=vgxc3x97xcfx84m greater than 1 cm. This disconnection provides the possibility for a soft surface cleaning avoiding destructive direct contact of plasma with patterns developed on the surfaces treated for semiconductor, display (Indium-Tin-Oxide), and micromachining technology.
The process of modification of the surface by its exposure to a flow of metastable atoms (molecules) mixed with a neutral gas and plasma can be realized with RF Capacitively Coupled Plasma (CCP) sources described in xe2x80x9cIndustrial Plasma Engineeringxe2x80x9d, Vol. 1, Principles, J. Reece Roth, Institute of Physics Publishing, 1995, p.p. 417-463. As seen in FIG. 12.12, page 443 of this book (see FIG. 1 of the present disclosure), a plasma source with outer ring type electrodes 2 and 3 is mounted outside of a quartz wall (or ceramic) tube 1 serving as a plasma reactor is one of the possible RF plasma sources applicable for metastable excited atomic (molecular) medium generation. This arrangement of the electrodes enables one to avoid plasma contamination with an electrode material affecting in general the life time of the metastable medium. The operating gas (or gas composition) required for such a process is fed to one of the ends of the tube 1 under necessary pressure, whereby, at the supply of RF power to said electrodes 1 and 2 at a frequency of 1 to 100 MHz, plasma 4 mixed inside the tube 1 with neutral gas and neutral metastable atoms (molecules) spreads along this tube due to the electromagnetic field action and causes a gas flow through the inter-electrode space to contact with a work piece 5 and to deliver in this manner to the work piece 5 surface the excited metastable atomic (molecular) medium produced from the operating gas (or gas composition).
A process of surface modification can be provided by static exposure of the work piece 5 surface to a flow of a working mixture of plasma with a neutral gas and an excited atomic (molecular) medium during a certain exposure time xcfx84ex, or by moving the work piece with a constant velocity vwunder the flow of this mixture that assumes the exposure time defined by the equation xcfx84ex=a/vw, where a is the diameter of the mentioned beam, see FIG. 1.
However a practical application of the RF source type presented in FIG. 1 for production of an excited to metastable state medium separated from plasma has problems. A combined plasma electrode configuration forms a typical dipole antenna propagating the electromagnetic field in surrounding space (as is seen in FIG. 1). This propagation promotes in turn plasma spread along the tube 1 to distances of several inter-electrode lengths. As a result, the plasma has a natural slow decrease in density outward from the source electrodes 2 and 3. Therefore at distance lw where this density is negligibly small and undesirable RF-arc breakdown from plasma to the surface of the workpiece becomes completely impossible, the density of the excited metastable medium (having a limited life time) produced by a weak plasma wing is negligible as well. To improve the output density of the excited metastable medium without jeopardizing the surface to be treated to RF-arc breakdown, one can increase the velocity of gas flow through the tube 1. However increasing gas velocity is very costly for an industrial process provided with operating gases having a relatively high price.
It is clear that the best way to increase the output density of the metastable excited medium is to form a clear cut sharp boundary of high density plasma downstream. Then at a distance safe for RF breakdown between the plasma boundary and the workpiece surface, one can realize a maximum possible excited metastable medium density provided by this close distanced high density plasma. To form the sharp plasma boundary at the metastable excited medium outflow, a propagation of the RF electromagnetic field should be cut sharply in the vicinity of the output electrode 3. This sharp cut of the electromagnetic field propagation is provided reliably in a device configuration described in a US Patent Application entitled RF LOADED-LINE TYPE CAPACITIVE PLASMA SOURCE FOR BROAD RANGE OF OPERATING GAS PRESSURE, application Ser. No. 10/192,329, Filed Jul. 10, 2002, see FIG. 2 of the present disclosure. In FIG. 2, an operating tube 1 of a high temperature insulator (quartz, ceramic) passes through a high voltage ring or collar type electrode 2 and further through a collar type grounded electrode 3 extended to form an outside cylindrical shield 7 enveloping coaxially the inner part of the operating tube 1 and the high voltage electrode 2. The outside cylindrical shield 7 is flanged, and the high voltage electrode 2 is mounted with this flange 7a by a cylindrical insulator 9. All the connections of the operation tube 1 with the electrodes 2, 3, the connections of the flange 7a with the cylindrical shield 7 and the insulator 9, as well as the connection of the insulator 9 with the high voltage electrode 2, are sealed by corresponding O-rings shown in FIG. 2 as solid black cross sections. To ignite the device, the special igniting electrode 6 is mounted at the output end of the operating tube 1. An RF power supply is connected with the device by a coaxial cable 11 and further by a water cooled conductive clamp 10 providing cooling of the high voltage electrode 2 Cooling of the grounded electrode 3 and the outside cylindrical shield 7 is provided by a water passage or conduit in their common body. A gaseous working composition is supplied via an end of the operating tube 1 from the high voltage electrode 2 side, and an excited to metastable state gaseous medium 4a produced by plasma 4 as generated by RF power applied between the electrodes 2 and 3 yields from the opposite end of the operating tube 1 with a gas flow in contact with a workpiece 5. The interior cavity 8 formed between the inner surface of the shield 7, the outer surfaces of the electrodes 2, 3 and the operating tube 1, and the inner surface of the flange 7a and the insulator 9, is filled through a pipe 12 with a special gas composition under a necessary pressure (or with a corresponding oil) to prevent RF discharge outside the operating tube 1, e.g. in the interior cavity 8. It is understood that for any reasonable pressure and gas (vapor) composition feeding the RF discharge plasma in operating tube 1, one can find a suitable medium along with its required pressure for filling interior cavity 8 to prevent direct RF breakdown between the electrodes 2 and 3. Intensive cooling of the outer surface of the operating tube 1 in the cavity 8 can be provided with application of an additional pipe, similar to pipe 12, installed at a proper place and connected in common with the pipe 12 to a heat exchanger if necessary.
The device embodiment shown in FIG. 2 is a terminating portion of the RF coaxial loaded line source where the high voltage electrode 2, plasma beam 4, and the inner part of the electrode 3 forms a core of this coaxial line. Therefore an electromagnetic field of this coaxial line exists only inside the cavity 8 at the proper connection of the RF power supply cable 11 to the device. In practice, plasma generated in the tube 1 of this device forms a clear cut sharp boundary shifted downstream from the grounded electrode 3 lower surface to a 2-3 tube diameter distance, depending on the RF power absorbed in plasma (cf. 2-3 inter-electrode distances for the device shown in FIG. 1).
A process for surface modification with application of the device shown in FIG. 2 can be provided by statically exposing the work piece 5 surface to a flow of an excited atomic (molecular) medium during a certain exposure time xcfx84ex, or by moving a work piece with the constant velocity vw under the flow of this medium that assumes the exposure time defined by the equation xcfx84ex=a/vw, where a is the diameter of a spot formed by the yielded flow on the surface treated, see FIG. 2.
It is an object of the present invention to provide a process of surface modification by exposing it to a flow of neutral atoms or molecules produced in plasmas excited to a metastable level. Another object of the invention is to provide a process of cleaning the surfaces without destruction of patterns built on the surfaces for semiconductor, display, or micromachining technologies. It is also an object of the invention to provide a process of selective modifications in thin surface layers of solids for the purpose of developing an adhesion bonding, sterilization or selective chemical and crystalline structure transformation of the surface by application of a specially chosen working gaseous medium having a certain required value of atom excitation energy.
The process of surface modification of the present invention is provided by exposing the surface to excited metastable atoms (or molecules) flowing to the surface being treated from a source of these atoms (or molecules).
To provide the certain selective surface modification process of the present invention, one can choose a gas or vapor with the required value of a metastable level energy within a variety of available metastable levels of existing gases and vapors.
To produce the excited metastable atoms (or molecules) for the surface modification process, the working gas medium is blown preliminary through a discharge gap filled with plasma generated of or from this gas medium.
To provide nondestructive cold surface modification or a corresponding cleaning process, an excited metastable atomic (or molecular) component is separated from plasma by shifting it from a plasma boundary to a distance d where a surface to be treated is located. A flow velocity vg of the gas medium throughout the discharge gap and further toward the surface being treated is set in this case to the value defined by the expression: vgxe2x89xa7d/xcfx84m, where xcfx84m is the life time of the working metastable level.
To provide the necessary energy and concentration of electrons in plasmas for effective excitation of a chosen metastable electron level of a primary operating gas, the binary or triple gas composition is developed, wherein secondary gases control key parameters of the gas discharge plasma.
An operating medium (gaseous or vapor) required for a specific process has a pressure from several mTorr to several atmospheric pressures. The density of electric power applied to plasma for an operating component excitation has a rating from tens of W/cm3 to 1000 W/cm3.